1. Field of the Invention
The present invention relates to an inflatable cuff for use in measuring a blood pressure of a living subject by detecting a Korotkoff sound produced from an artery of the subject.
2. Related Art Statement
There are known two sorts of blood pressure (BP) measuring devices each of a Korotkoff-sound type wherein an inflatable cuff having an elongate, belt-like configuration is wound around, e.g., an upper arm of a living subject such as a patient, and an air is supplied into the cuff to inflate the cuff and thereby press the upper arm, so that systolic and diastolic BP values of the subject are measured by detecting the first and last Korotkoff sounds produced from an artery of the arm while the air pressure of the cuff pressing the arm is slowly decreased. One of the two sorts of BP measuring devices is an automatic BP measuring device including a microphone, and the other is a mercurial manometer used with a stethoscope.
The Korotkoff sound-type BP measuring devices as described above are used for, e.g., measuring BP values of a subject who is undergoing an exercise test. The BP measurement is usually carried out using an automatic BP measuring device including a microphone. The microphone, however, may pick up noise such as frictional sounds of the cuff, so that the signal to noise (S/N) ratio of a Korotkoff-sound signal produced from the microphone may be lowered and accordingly the accuracy of BP measurements based on the Korotkoff-sound signal may be lowered or the BP measurements using the cuff may be made even infeasible. The noise may be removed from the Korotkoff-sound signal, by successively determining an appropriate time window or gate using an R wave of an electrocardiogram (ECG) obtained from the subject as a trigger to start each time gate, and collecting a Korotkoff sound through the thus determined each time gate.
However, even though the noise resulting from the subject's physical motion may be removed from the Korotkoff-sound signal, the accuracy of BP measurements based on the Korotkoff-sound signal may be lowered for other reasons. More specifically, if a certain force is applied to the cuff because of the subject's physical motion during the above-mentioned slow decreasing of the air pressure of the cuff currently pressing the subject' arm, the cuff pressure may considerably largely be changed. Those changes of the cuff pressure adversely influence the detected magnitudes of the Korotkoff sounds that are produced in relation with the state or degree of occlusion of the subject's artery, so that the first and/or last Korotkoff sounds may not be detected by the microphone with accuracy. Consequently the accuracy of BP measurements may be lowered or the BP measurements may be made infeasible. The problem that the changes of cuff pressure due to the subject's motion adversely influence the accurate detection of Korotkoff sounds and eventually the accuracy of BP measurements, will be encountered also in the case where the above-identified mercurial manometer is used with a stethoscope by a doctor or nurse.
A conventional cuff is provided by a cloth substantially incapable of stretching, for preventing an excessive amount of air from being supplied to the cuff. The present inventor has conducted an extensive study for solving the above-explained problem and found for the first time that a cuff capable of stretching advantageously absorbs the changes of cuff pressure due to the subject's physical motion.